Embodiments of the invention relate generally to transformers and, more particularly, to tap-changing circuits designed to change the transformer turns ratio to alter its output energy.
New battery technologies are emerging that can accept very high charge currents and thus can be charged in relatively fast times. These batteries may be used, for example, in electric and hybrid vehicle traction applications (e.g., passenger vehicles, busses, transit cars, on-road and off-road vehicles, golf cars, neighborhood electric vehicles, forklifts, utility trucks, as well as other higher power storage applications). Hybrid electric vehicles may combine an internal combustion engine and an electric motor powered by an energy storage device, such as a traction battery, to propel the vehicle. Such a combination may increase overall fuel efficiency by enabling the combustion engine and the electric motor to each operate in respective ranges of increased efficiency. Electric motors, for example, may be efficient at accelerating from a standing start, while combustion engines may be efficient during sustained periods of constant engine operation, such as in highway driving. Having an electric motor to boost initial acceleration allows combustion engines in hybrid vehicles to be smaller and more fuel efficient.
Purely electric vehicles use stored electrical energy to power an electric motor, which propels the vehicle and may also operate auxiliary drives. Purely electric vehicles may use one or more sources of stored electrical energy. For example, a first source of stored electrical energy may be used to provide longer-lasting energy while a second source of stored electrical energy may be used to provide higher-power energy for acceleration, for example.
Plug-in electric vehicles, whether of the hybrid electric type or of the purely electric type, are configured to use electrical energy from an external source to recharge the traction battery. These vehicles may use either off-board stationary battery chargers or on-board battery chargers to transfer electrical energy from a utility grid or renewable energy source to the vehicle's on-board fraction battery.
Conventional tap-changing circuits involve a primary winding of a transformer coupled to a switch array including back-to-back thyristors or switch pairs that are used to connect a primary transformer winding tap to an AC source. This has the effect of changing the transformer turns ratio to alter the output voltage and current as needed during battery charging. The current is limited by the transformer impedance (i.e., leakage inductance). The transformer can be designed to have a 15 to 20% impedance. When the current drops off as the battery voltage increases, the transformer tap can be changed to increase the current back to a higher level. However, the components of such conventional tap-changing circuits are often expensive and employ complicated control techniques.
It would therefore be desirable to provide a cost-effective and reliable apparatus to provide high currents to DC loads.